This invention relates to controlling electrostatic charge and voltage on a moving dielectric web, and more particularly to electrostatography and to apparatus and method for control of electrostatic charge and voltage on a receiver-transporting web in an electrostatographic printer.
Prior art discloses apparatus for applying corona charges to a moving web, or to sheets supported by a moving web. In many cases it is desirable to apply the corona charges by one or more corona chargers, with a purpose of neutralizing electric fields resulting from extraneous electrostatic charges residing on the surfaces of the web, or within the interior of the web. In other cases, it is required to produce a uniformly charged web, or a web having a uniform average voltage which may be positive, negative, or zero. Such a uniform average voltage may be characterized by a variance from a mean voltage, and the variance may for example be required to be less than a predetermined variance.
The Gibbons patent (U.S. Pat. No. 3,470,417) discloses electrical conditioning of a bare web by gridded corona chargers located on opposing sides of a moving dielectric web, each corona charger energized by a DC voltage, which electrical conditioning can produce a predetermined potential on each face of the web and can also be used to neutralize substantially all charge on a web.
The Kerr patent (U.S. Pat. No. 3,730,753) describes a method for removing a nonuniform charge distribution from a web that has previously been treated by an AC corona discharge for purpose of making the web coatable by an emulsion. The method involves flooding the corona discharge treated surface with negative charge by a high voltage negative DC non-gridded corona charger, followed by reducing the surface charge on the web to approximately zero by a high voltage positive DC non-gridded corona charger.
The Rushing et al. patent (U.S. Pat. No. 4,245,272) discloses a so-called xe2x80x9cboost and trimxe2x80x9d corona charging method for charging a moving dielectric film or web, e.g., a photoconductor. The xe2x80x9cboostxe2x80x9d produces an overcharging of the photoconductor at the beginning of the process of charging a given area of the film, and the xe2x80x9ctrimxe2x80x9d subsequently reduces this overcharge so as to give a predetermined exit voltage as the given area leaves the xe2x80x9cboost and trimxe2x80x9d charger. A xe2x80x9cboost and trimxe2x80x9d charger as described in U.S. Pat. No. 4,245,272 is a multiple open wire charger (no grid) with each wire energized by a DC-biased AC voltage source. Typically, an AC signal is applied in common to all wires of the charger, with a different DC potential applied to each wire. The waveform shape of the AC signal is not specified.
The Cardone patent (U.S. Pat. No. 4,486,808) discloses an open-wire (no grid) corona charger energized by an AC voltage and located on one side of a dielectric web, and an open-wire DC-biased AC charger located on the other side of the dielectric web. The waveform shape of the AC voltage is not specified.
The Inoue et al. patent (U.S. Pat. No. 4,737,816) discloses a detack charger assembly for neutralizing charges on a toned receiver member carried by a transport belt, which neutralizing allows the receiver member to be readily removed from the belt by a pawl. The detack charger assembly has two opposed corona chargers, and the toned receiver member on the transport belt is moved between them. Each of the chargers is energized by an AC voltage which may include a DC offset, the AC voltages being applied to the two chargers 180 degrees out of phase with one another. The waveform shape of each AC voltage is not specified. It is also briefly disclosed that a grid may be used on a charger to control the charging current.
In the Amemiya et al. patent (U.S. Pat. No. 4,914,737) a corona discharge device is used following a corona transfer device for transferring toner from a photoconductive primary imaging member to a receiver (paper), the receiver supported by a dielectric sheet member during both transfer of the toner and during operation of the corona discharge device. The corona discharge device includes two single-wire non-gridded corona chargers, i.e., an outer corona charger facing the toner on the front side of the receiver (after transfer of the toner from the primary imaging member to the receiver) and an inner corona charger facing the back side of the dielectric sheet member. An AC voltage is applied to both corona chargers, the AC voltages being out of phase with one another. The waveform shape of each AC voltage is not specified. An appropriate DC bias voltage may be applied to either or both of the outer and inner corona chargers.
The Takeda et al. patent (U.S. Pat. No. 5,132,737) discloses a pair of single-wire non-gridded corona dischargers (voltage excitation waveforms not specified) for post-transfer use with a dielectric carrying sheet supporting a toned transfer material such as paper, with one of the corona dischargers disposed facing the toned transfer material and the other corona discharger disposed facing the back side of the dielectric carrying sheet.
The Amemiya et al. patents (U.S. Pat. Nos. 5,589,922 and 5,890,046) disclose opposed open-wire non-gridded corona discharge devices, disposed similarly to the open-wire corona discharge devices of the Amemiya et al. patent (U.S. Pat. No. 4,914,737) and similarly employing mutually out-of phase AC voltage waveforms including DC offsets, certain embodiments using plural corona wires. The AC waveform shapes are not specified.
A commercial corona discharger assembly for neutralizing static charges on both sides of a dielectric web is manufactured by HAUG GmbH of Leinfelden-Echterdingen, Germany. An AC Power pack (catalog number EN-70 LC) is utilized for energizing four xe2x80x9cionizing barsxe2x80x9d (catalog number EI-RN), the ionizing bars mounted as two successive pairs, one ionizing bar of each pair disposed on either side of a dielectric web, each ionizing bar powered by an AC sinusoidal waveform such that the two waveforms of each pair are 180 degrees out of phase. No DC offset biases are specifically described, nor are grids included with the ionizing bars.
Several commercial electrophotographic printing machines (e.g., Xerox Docucolor 40, Ricoh NC 8015, Canon CLC 1000) employ an endless insulating transport belt for carrying receivers through multiple successive transfer stations so as to build up a multicolor toner image on each receiver, in which machines the endless transport belt, after detack of the receivers, is passed through a charging apparatus for neutralizing unwanted surface charges and/or for use as a pre-clean charging station prior to cleaning the transport belt. In a Xerox Docucolor 40, a pair of opposed single wire AC pre-clean corona chargers having metal shells and no grids are disposed on opposite sides of the transport belt, the chargers using square wave excitation at a frequency of about 1000 Hz. A Ricoh NC 8015 machine uses an open-wire AC charger on the front side of the transport belt, the charger opposed by a roller on the back side of the belt. The Canon CLC 1000 machine includes a detack station which detack station includes a DC-biased open-wire AC charger opposed by a roller, a post-detack roller nip having grounded rollers through which the transport web passes so as to even out the potential differences between frame and interframe areas, with the post-detack roller nip followed by a back-side web cleaner that also functions as a static charge eliminator.
The Gundlach et al. patent (U.S. Pat. No. 6,205,309) discloses an AC corona charger wherein a corona wire is coupled through a capacitative connection to an AC voltage source, the corona wire partially surrounded by a conductive shield connected to a DC voltage source. The presence of a capacitance between the AC voltage source and the corona wire ensures that equal numbers of positive and negative corona ions are generated at the wire, with the DC potential controlling the net charging current, e.g., for purpose of charging a photoconductive member. It can be inferred that by setting the DC potential close to zero, the charger may be used as a neutralizer.
There remains a need for an improved non-contacting web conditioning charging apparatus for effectively removing nonuniform charge distributions from a moving dielectric member, e.g., for neutralizing extraneous electrostatic charges on the front and back surfaces of a moving dielectric web, where the incoming web entering the charging apparatus may have a potential difference across the web of thousands of volts, e.g., 4,000 volts or higher across a 100 xcexcm thick web. In particular, there remains a need for an improved corona charging device having high reliability and robustness for the smoothing or neutralizing of nonuniform electrostatic charge distributions on the surfaces of a rapidly moving transport web, such as a transport web used for transporting receiver members through successive imaging modules of a modular electrostatographic color printing machine. There is an additional need for the neutralizing or smoothing to be carried out on the entire operational area of such a transport web, the operational area including area portions of the web from which toned receiver members have been detached, e.g., without the use of a detack charger.
Accordingly, this invention is directed to a robust and reliable web conditioning charging station (WCCS) for repeatably controlling electrostatic charge and voltage on a moving dielectric web. In a preferred embodiment, the WCCS is used for neutralizing electrostatic charge. In another embodiment, the WCCS is used for providing a predetermined, uniform, potential difference across the web. The WCCS has a first stage and a second stage through which the web moves. The first stage includes an opposed pair of open wire (no grid) corona chargers facing one another on opposite sides of the web, each charger energized by an AC voltage waveform and the two waveforms mutually 180xc2x0 out of phase. The second stage includes a second pair of corona chargers, each charger of the second pair provided with an electrically biased grid member and the chargers facing one another on opposite sides of the web, with each charger of the second pair energized by an AC voltage waveform and the two waveforms of the second pair mutually 180xc2x0 out of phase. In a preferred embodiment, for use to condition a transport web (TW) in an electrophotographic color printer optionally having no detack charger, the AC voltage waveforms for energizing the first and second stages are quasi-trapezoidal with zero DC offsets, the waveforms of the first and second stage chargers are in phase on either side of the web, and the grid members of the second pair of chargers are at ground potential.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.